For some years, we have been using the unique tool of inbred histocompatible guinea pigs for investigation of mechanisms of hypersensitivity for protection against infectious and autoimmune diseases. Advantages lie in the ability to perform viable adoptive transfer of lymphoid cells, and in the capacity to study genetic factors in certain hypersensitivity and autoimmune phenomena. We have concentrated recently on the pathogenesis of an age-dependent, strain-dependent chronic relapsing autoimmune encephalomyelitis which is strongly reminiscent of multiple sclerosis in man, and on the prevention and treatment of this condition. Chronic EAE has been suppressed by appropriate injection of myelin basic protein and the protection monitored by circulating T cell changes as well as by clinical signs and pathological examination.